The present invention relates to barium- and strontium-free jointing materials which, in particular, are suitable for sealing high-temperature fuel cells or electrolysis cells, and uses thereof.
For the purposes of the invention, jointing materials are materials which, starting from a glass material, can be present predominantly in amorphous, partially crystalline and/or crystalline form. These are therefore referred to as vitreous or glass-ceramic jointing material for the purposes of the invention. These jointing materials can be used as glass solders, but also in the form of preforms, etc.
Vitreous or glass-ceramic jointing materials are usually used for producing joint connections, in particular, in order to join glass and/or ceramic components to one another or to components composed of metal in an electrically insulating manner. In the development of jointing materials based on glass, their composition is often selected so that the coefficient of thermal expansion of the jointing materials corresponds approximately to that of the components to be joined to one another in order to obtain a lastingly stable joint connection. Compared to other joint connections, for example those composed of plastic, jointing materials based on glass have the advantage that they form a hermetic seal and can withstand higher temperatures.
Jointing materials in the form of glass solders are generally often produced from a glass powder which in the joining operation, also known as soldering operation, is melted and together with the components to be joined gives the joint connection under the action of heat. The joining temperature is generally selected so as to correspond approximately to the spherical temperature of the glass. Measurement of the spherical temperature is a standard measurement method known to those skilled in the art and can be carried out using a hot-stage microscope. If a crystallization-free glass in the form of a glass powder is melted as jointing material and cooled again so that it solidifies, it can usually be melted again at the some melting temperature. In the case of a joint connection composed of a jointing material in the amorphous state, this means that the operating temperature to which the joint connection can be subjected to in the long term must not be higher than the joining temperature. In actual fact, the operating temperature in many applications has to be significantly below the joining temperature since the viscosity of the jointing material decreases with increasing temperature and a glass having some flowability can be squeezed out of the joint connection at high temperatures and/or pressures, so that the joint connection can fail in its function. For this reason, jointing materials based on glass for high-temperature applications usually have to have a joining temperature which is significantly above the later operating temperature. This is possible in the case of non-crystallizing jointing materials based on glass, i.e. those which are present as amorphous jointing material before the joining operation, or else by means of at least partially crystalline jointing materials in the case of which the base glass crystallizes at least partially or else completely during the joining operation. For the purposes of the invention, partially or completely crystallized jointing materials are referred to as glass-ceramic jointing materials or glass-ceramic.
For particularly high use temperatures, ceramic materials in the case of which an at least substantially amorphous base glass crystallizes at least partly or else completely during the joining operation are frequently used as jointing material. The crystalline phases or the ceramics generally have properties which are significantly different from those of the amorphous base glass, e.g. in respect of the thermal expansion or the glass transition temperatures, on that the total system composed of amorphous glass phase and crystalline phases can likewise have properties which are different from those of the amorphous base glass alone. In particular, in the case of glass-ceramic jointing materials the temperature required for the melting can be significantly above that for the amorphous base glass. Whether an amorphous glass or a glass-ceramic is formed from an amorphous base glass during the joining operation depends, in the case of a suitable composition of the base glass, firstly on the way in which the joining operation is carried out, in particular on the heating and cooling curves. The term vitreous or glass-ceramic jointing material therefore encompasses, for the purposes of the present invention, both the base glass and also the system formed therefrom after use, depending on whether it is amorphous, partially crystalline and/or crystallizes completely.
One field of use of jointing materials having a high melting temperature is, for example, joint connections in high-temperature fuel cells which can be used, for example, as energy source in motor vehicles or for decentralized energy supply. An important fuel cell type is, for example, the solid oxide fuel cell (SOFC) which can have very high operating temperatures of up to about 1100° C. The joint connection with the jointing material is usually used for producing fuel cell stacks, i.e. for joining a plurality of individual fuel cells to form a stack. Such fuel cells are already known and are continuously being improved. In particular, the trend in present-day fuel cell development is generally towards lower operating temperatures. Some fuel cells reach their operating temperatures below 800° C., so that lowering of the joining temperatures is possible and also desirable because of the lower thermal stress on the SOFC components during the joining operation.
Electrolysis cells have a structure similar to that of fuel cells, especially solid oxide electrolysis cells (SOECs) which can be used for preparing chemical elements and/or compounds and can play a role in, for example, the storage and/or conversion of energy produced on a renewable basis. These are likewise a preferred field of use of the jointing materials of the invention.
A further field of use of the jointing materials described here are any components, e.g. sensors and/or actuators, which are subjected to high temperatures. Examples of uses are in the exhaust gas train of an energy generation unit or in the combustion chamber itself. The energy generation unit can be, for example, an internal combustion engine, an aircraft turbine, a gas turbine, etc. The jointing materials are in these cases often used in the housing of these sensors and/or actuators, for example in order to join housing parts to one another or realize electric leads passing through the housing. In these applications, operating temperatures of more than 800° C., even more than 1000° C., are often exceeded. Applications in the field of solar energy generation, e.g. in solar furnaces, or for lead-throughs in particularly critical fields, e.g. nuclear power stations, fusion power stations, etc., are likewise possible.
Even higher operating temperatures of above 1000° C. are required in the case of components in which individual components made of ceramic have to be joined.
The possible operating temperature of vitreous or glass-ceramic jointing materials, and also their chemical properties associated therewith and their coefficient of thermal expansion, are important criteria which qualify the jointing materials for the intended applications. The chemical properties of the jointing material should be compatible with the material which is joined by the jointing material, in particular the operating temperature, and likewise with the surroundings. For example, vitreous or glass-ceramic jointing materials should often be so chemically resistant that they can withstand the materials and/or mixtures used or formed in the fuel cells or electrolysis units and also further reaction products in the long term.
Vitreous or glass-ceramic jointing materials per se are known from numerous publications. However, few are suitable for high-temperature applications.
DE 600 25 364 T2 describes a glass-ceramic composite which consists of the system BaO—SrO—CaO—MgO—Al2O3—SiO2. Glass-ceramic compositions comprising at least 20 mol % of BaO and up to 20 mol % of B2O3 are disclosed.
DE 10 2005 002 435 A1 relates to glass-ceramics as jointing material for high-temperature applications. The material system likewise allows significant amounts of BaO and contains at least 15% by weight of B2O3. No information is given regarding the physical properties.
U.S. Pat. No. 6,532,769 B1 describes a join comprising a glass containing at least 20 mol % of BaO.
All these glasses and glass-ceramics can contain appreciable amounts of BaO. BaO is used in the art as constituent of such materials in order to set the desired high coefficient of thermal expansion. Apart from the glass former SiO2, BaO is thus a main component of the glass or the glass-ceramic. However, all these glasses containing barium oxide suffer from the disadvantage that they undergo interfacial reactions with chromium and thus have only a low utility and poor adhesion to chromium-containing materials. Their thermal cyclability is likewise adversely affected thereby. As a result, joint connections comprising materials having a large chromium content, e.g. chromium steels and chromium-nickel steels, and these glasses are unstable, i.e. the glass layer easily flakes off the materials having a high chromium content.
Barium-free glass solders are known from U.S. Pat. No. 7,214,441 B2. Instead of BaO, the description provides for SrO in amounts of from ≧10 to 25 mol %. The examples have a minimum SrO content of 18 mol % or the only examples which do not contain SrO have B2O3 as glass former in large amounts of more than 40 mol %. High contents of B2O3 significantly decrease the chemical resistance of the glasses, so that they cannot be used in the long term in the environments typical for high-temperature applications, in which aggressive media and/or aggressive substances usually occur. In addition, a poor chemical resistance in contact with chromium-containing alloys is also observed as a result of the high content of SrO.
It has been found that joint connections which are stable in the long term cannot be produced using these and the SrO-containing variants.